Information is written onto magnetic media by magnetizing regions with a particular strength and orientation. These regions generate a magnetic field which can be converted into an electrical signal by sensors located in a read head. A common type of read sensor, adaptable to small geometries through thin film processing techniques, exhibits a change in resistance to varying strengths of magnetic fields. Small read sensor geometries are necessary in achieving increasingly greater information areal density on magnetic media.
A simple magnetoresistive (MR) sensor consists of a thin film of magnetoresistive material, such as Permalloy, between two insulating layers. When the MR layer is formed, a magnetic field is typically applied in a direction parallel to the plane of the thin layer. Thus, the MR layer exhibits a uniaxial anisotropy with an easy-axis of magnetization parallel to the direction of the applied field. If an external magnetic field, such as produced by magnetized regions on moving magnetic media, is applied normal to the easy-axis, the magnetization direction of the MR layer will rotate away from the easy-axis and towards the direction of the applied magnetic field. This magnetization rotation causes a change in resistance in the MR layer. To compensate for non-linearities, the MR sensor is often biased with an applied current such that a zero magnitude applied field results in a resistance near an inflection point on the resistance curve. Thus, small changes about a zero magnitude applied external field result in nearly linear changes in resistance. One difficulty with this simple MR design is the low percentage change in resistance for the level of flux commonly exhibited by moving magnetic media. This low sensitivity results in a high susceptibility to noise and limits the reduction of track size on the magnetic media.
Greater sensitivity may be achieved using a laminate structure of at least two uncoupled ferromagnetic layers with each pair of ferromagnetic layers separated by a non-ferromagnetic metal layer. The scattering of charge in current flowing through the structure is dependent on an induced spin direction. The giant magnetoresistive (GMR) effect results from varying the spin direction in one of the ferromagnetic layers. A read sensor results when the orientation of the free ferromagnetic layer is changed due to externally applied fields from the passing magnetic media.
An application of the GMR effect is the spin valve magnetoresistive (SVMR) sensor. In an SVMR sensor, one ferromagnetic layer is pinned. This pinning may be accomplished by placing an anti-ferromagnetic layer in contact with the pinned ferromagnetic layer. Many designs for SVMR sensors exist, including those described in U.S. Pat. Nos. 5,206,590 to Dieny et al. and 5,465,185 to Heim et al, both of which are incorporated by reference herein. In order to be effective, the SVMR sensor must possess several qualities. First, the exchange bias between the pinned ferromagnetic layer and the pinning anti-ferromagnetic layer must be great enough to rigidly pin the ferromagnetic layer against the small field excitations produced by the media and large demagnetizing fields resulting from the sensor geometry. Second, the device must have a low coercivity to prevent the pinned layer from becoming unstable in the presence of moderately strong applied fields. Third, if the magnetic media comes into contact with the read sensor as with magnetic tape, materials used in the sensor must have good wear properties.
Two SVMR sensor designs are described in U.S. Pat. No. 5,701,223 to flontaiia, Jr. et al. which is incorporated by reference herein. Both designs use a synthetic anti-ferromagnet as the pinned layer. A synthetic anti-ferromagnet is an ultra-thin laminate structure of coupled ferromagnetic films, each neighboring pair separated by a non-magnetic coupling film. By adjusting the thickness of the coupling film, the coupled ferromagnetic films become alternately aligned and anti-aligned. For example, a synthetic anti-ferromagnetic layer with anti-aligned ferromagnetic films may be constructed of ultra-thin layers of cobalt (Co) separated by a ruthenium (Ru) layer 2-8 .ANG. thick. A pinned synthetic anti-ferromagnet is formed by placing an anti-ferromagnetic layer, such as nickel oxide (NiO) or iron-manganese (Fe--Mn), adjacent to one of the ferromagnetic films. Both designs disclosed utilize a pinned synthetic anti-ferromagnet. The designs differ in the order in which layers are formed and the material used for tile anti-ferromagnetic pinning layer.
The first design describes a bottom SVMR. In one embodiment, an NiO anti-ferromagnetic layer is deposited on a substrate layer. A synthetic anti-ferromagnet, consisting of an ultra-thin layer of Ru between ultra-thin layers of Co, is deposited on the NiO to form a pinned synthetic anti-ferromagnet. A spacer layer, such as copper (Cu), is deposited on the synthetic anti-ferromagnet. A nickel-iron (Ni--Fe) ferromagnetic layer is deposited on the spacer. An advantage with this design is the good wear properties of materials used. One problem with this design is the difficulty in achieving proper soft magnetic properties such as low coercivity, low anisotropy, and a well-defined magnetic axis in the free ferromagnetic layer without previously depositing a seed layer directly beneath the free layer. Another problem is the difficulty controlling the structure and surface quality of the NiO anti-ferromagnetic layer.
The second design described is the top SVMR sensor. A tantalum (Ta) seed layer is first deposited on the substrate. The Ni--Fe free ferromagnetic layer is deposited on the seed layer. A spacer layer is then deposited on the free ferromagnetic layer. The pinned synthetic anti-ferromagnet is then formed on the spacer. To make the pinned synthetic anti-ferromagnet, a Co/Ru/Co synthetic anti-ferromagnetic layer is first deposited. An Fe--Mn anti-ferromagnetic layer is then deposited on the synthetic anti-ferromagnetic layer. The use of a seed layer maintains the soft magnetic properties of the NiFe free ferromagnetic layer. However, the Fe--Mn used for the pinning anti-ferromagnetic layer has inferior wear properties and corrosion resistance.
What is needed is a synthetic anti-ferromagnet that is easy to produce using thin-film manufacturing techniques. A top SVMR should be able to be produced with this pinned synthetic anti-ferromagnet that exhibits a high exchange bias between the synthetic anti-ferromagnet and is constructed of materials with good wear properties and corrosion resistance.